Transcript VLSM
VLSM and CIDR Review CSE 421 CCNA Exploration 2 Chapter 6 1 Thanks to S Ward Abingdon and Witney College Topics Revision of classful and classless IP addressing Revision of VLSM and benefits Use of Classless Interdomain Routing (CIDR) 2 Classful addressing 3 Network part and host part 4 Classful networks Address class First octet range Number of networks Hosts per network Class A 0 to 127 128 (less 0 and 127) 16,777,214 Class B 128 to 191 16,348 65,534 Class C 192 to 229 2,097,152 254 5 IPv4 Address Exhaustion 6 Solutions Long term: Change to IP version 6. Plenty of addresses using a different scheme Short term: Use VLSM and CIDR to avoid wasting addresses Use private addresses locally and NAT for internet access – lets many hosts share a few public addresses 7 Classful Subnetting All subnets of a main network must have the same subnet mask. This means that they must all have the same number of hosts. 8 Classful Subnetting 192.168.80.192/27 192.168.80.0/24 192.168.80.32/27 192.168.80.160/27 192.168.80.0/27 192.168.80.128/27 192.168.80.64/27 192.168.80.96/27 9 Subnet 192.168.1.0/24 10 hosts 26 hosts 12 hosts Need 6 networks, up to 26 hosts. Borrow 3 bits, /27, 255.255.255.224 Gives 8 networks, up to 30 hosts. Point to point need 2. 28x3 = 84 wasted 10 Subnet 172.16.0.0/16 100 hosts 500 hosts 350 hosts Need 6 networks, up to 500 hosts. Borrow 7 bits, /23, 255.255.254.0 Gives 128 networks, up to 510 hosts. Point to point need 2. 508x3 = 1524 wasted 11 Waste Classful subnetting wastes addresses. If you are using private addresses then you may not be bothered. Waste of public addresses does matter. 12 Solution: Variable Length Subnet Masking (VLSM) Create subnets as per specific host requirements. Need 10 addresses /28. Give them 16. Need 500 addresses /23. Give them 512. CSE 421 13 Variable Length Subnet Masking (VLSM) “If you know how to subnet, you can do VLSM.” What’s the trick? Always satisfy the requirements of your biggest LAN and then work your way down …. 14 Variable Length Subnet Masking (VLSM) 15 VLSM 201.1.0.0/22 201.1.4.0/23 Advertise? Router A 201.1.6.0/24 201.1.7.0/24 How many entries does Router A has in it’s routing table? 16 Route summarization Octet 3 in binary 201.1.0.0/22 201.1.4.0/23 201.1.6.0/24 201.1.7.0/24 00000000 00000100 00000110 00000111 Same Same Difference starts here Difference starts here 21 bits the same so use /21 for summary 17 Route summarization 201.1.0.0/22 201.1.4.0/23 Advertise 201.1.0.0/21 201.1.6.0/24 201.1.7.0/24 •A Supernet summarizes multiple network addresses with a mask that is less than (or a summary of) the classful mask. 18 Route summarisation What address would summarise: 170.16.0.0/16 170.17.0.0/17 170.17.128.0/17 15 the same altogether 170.16.0.0/15 Octet 2 in binary 00010000 00010001 00010001 7 the same here You can create static summary routes. Dynamic routes can be summarised. 19 CIDR Concept • Classless InterDomain Routing (CIDR) introduced 1993 by IETF. To CIDR-compliant routers, address class is meaningless. The network portion of the address is determined by the network subnet mask, also known as the network prefix, or prefix length (/8, /19, etc.). 20 Classless Routing Protocols • Classless routing protocols are complaint with CIDR which means they understand • • • VLSM and Route Summarization. All routing protocols are classless nowadays. 21 VLSM Exercise Subnet 172.16.0.0/16 by borrowing 4 bits. Then subnet the third subnet by borrowing 2 more bits. Write out the subnet addresses and masks. 22 Subnetting 172.16.0.0/16 172.16.0.0/20 172.16.128.0/20 172.16.16.0/20 172.16.144.0/20 172.16.32.0/22 172.16.32.0/20 172.16.36.0/22 172.16.48.0/20 172.16.40.0/22 172.16.64.0/20 172.16.44.0/22 172.16.160.0/20 172.16.176.0/20 172.16.192.0/20 172.16.80.0/20 172.16.208.0/20 172.16.96.0/20 172.16.224.0/20 172.16.112.0 /20 172.16.240.0 /20 23 The End 24